Menu: View->Clip planes...
This window lets you hide the front or back of the view.
Clip planes is a way to prevent GiD from drawing elements of the geometry or mesh that are either very close to or far from the observer.
By moving the Near plane ( blue square in the sketch), the geometry that is closer to the viewer is clipped away, while moving the Far plane ( red square in the sketch) culls the geometry that is further from the viewer.
Selecting Move planes together leaves a constant distance between both planes, and so moving one plane it moves de second one at the same time.
Note: All this information is reset when performing a zoom frame. (see Zoom).